Dynamo electric machines, and specifically alternating current generators, are widely used in motor vehicles, in which, customarily, the alternator is driven from the crankshaft of a piston-cylinder-type internal combustion engine by drive belts or the like. Alternators of this type are subject to wide speed variations, and thus must be capable of providing output within the entire range of drive speed. The drive speed variation to which such alternators are subjected may extend from idling speed, about 400 to 600 rpm, to about 6000 rpm. The speed range within which such alternators operate, therefore, can change in a ratio of 1:10; usual ranges of variation of speed between idling and maximum speed of the engine are in the order of about 1:6. The speed range within which the alternator must operate thus is extremely broad.
Alternators of the automotive type are usually supplied with field current over slip rings, which energize a field of the alternator with current pulses to provide the requisite voltage output within ranges which can be accepted by a floating vehicle battery. Frequently, the magnetic poles of the field winding are determined by the physical structure of the field, or the magnetic conditions within the range of the rotor. Interdigited or claw pole-type machines are frequently used. By control of the current through the field, the power supplied by the alternator is matched to the speed with which the alternator is driven at any instant. The alternators themselves, while of simple and rugged construction, still are subject to operating wear and tear; the slip rings, as well as the current supply brushes which connect field current to the field winding, introduce a loss in power and may be the cause of malfunction and maintenance problems.
Various types of internal combustion engines, both of the Otto and of the Diesel type, are equipped with turbo-chargers. One such type is an exhaust gas operated turbo-charger, in which the energy still available from exhaust gases is utilized to drive a compressor which supplies precompressed air to the intake of the internal combustion (IC) engine. This exhaust gas turbine-turbo-charger compressor usually is a single-stage turbine receiving its drive energy from the exhaust gases of the associated IC engine. The compressor itself customarily is constructed in form of a radial blower or radial compressor which has a common shaft with the turbine blade wheel. Such exhaust gas operated precompressors or superchargers operate at very high speed; depending on the construction type, they may operate between 20,000 and 80,000 rpm. Under normal operation, however, the variation in speed, that is, the range of speeds with which such turbines and superchargers operate is much less than the speed range of the IC engine itself. Further, the turbine-supercharger combination is rarely subject to rapid changes in speed, as is an IC engine for vehicular use, for example operating between idling and close to maximum speed within a very short interval if it is desired to rapidly accelerate a vehicle from standstill to highway cruising speed.